--emit=llvm-bc in workspaces

[purpleposeidon]

$ export CARGO_INCREMENTAL=1
$ cargo clean
$ cargo rustc -p header -- --emit=llvm-bc
   Compiling header v0.1.0 (/media/media/rust/plugins3/header)
    Finished dev [unoptimized + debuginfo] target(s) in 0.42s
$ cargo rustc -p plugin -- --emit=llvm-bc
   Compiling header v0.1.0 (/media/media/rust/plugins3/header)
   Compiling plugin v0.1.0 (/media/media/rust/plugins3/plugin)
    Finished dev [unoptimized + debuginfo] target(s) in 0.24s
$ cargo rustc -p header -- --emit=llvm-bc
   Compiling header v0.1.0 (/media/media/rust/plugins3/header)
    Finished dev [unoptimized + debuginfo] target(s) in 0.11s
$ cargo rustc -p plugin -- --emit=llvm-bc
   Compiling header v0.1.0 (/media/media/rust/plugins3/header)
   Compiling plugin v0.1.0 (/media/media/rust/plugins3/plugin)
error: could not copy "/media/media/rust/plugins3/target/debug/deps/plugin-b8dfbf0a2496ce43.1qo0jbf99rserog8.rcgu.bc" to "/media/media/rust/plugins3/target/debug/deps/plugin-b8dfbf0a2496ce43.bc": No such file or directory (os error 2)

error: could not compile `plugin` due to previous error
$ rustc --version
rustc 1.55.0 (c8dfcfe04 2021-09-06)

I have a git repo that can easily demonstrate this issue; see incr-test in this repo.

It doesn't happen w/o incremental compilation.

(Search terms: --emit emit --emit=llvm-bc --emit=llvm-ir llvm-bc llvm-ir)

[BGR360]

Potentially the same thing as #89149

[hydra]

I tested #89149 today with out incremental compilation and found that the issue there only happens /with/ incremental compilation, the same as in this issue.

[clubby789]

@rustbot label +A-incr-comp +C-bug